1. Field of the Invention
The present invention relates to a honeycomb filter and an exhaust gas treatment apparatus, further particularly to a honeycomb filter capable of inhibiting an end face on an inflow side of a fluid from being closed by particulates and the like and maintaining high strength, and an exhaust gas treatment apparatus having the honeycomb filter.
2. Description of the Related Art
There has been an increasing necessity for removing particulates or toxic substances from an exhaust gas of an internal combustion engine, boiler or the like in consideration of influences on environment. Especially, there has been a tendency that regulations on removal of particulate matters (hereinafter referred to also as “PM”) discharged from a diesel engine have been strengthened worldwide, use of a honeycomb filter has been noted as a diesel particulate filter (hereinafter referred to also as “DPF”) for removing the PM, and various systems have been proposed. The DPF usually has a structure in which a plurality of cells constituting fluid channels and having square sectional shapes are partitioned/formed by porous partition walls, and the porous partition walls constituting the cells perform functions of filters by alternately plugging the cells. Here, the sectional shape refers to the shape of the section obtained by cutting the cell with a flat face vertical to a longitudinal direction.
The DPF allows particulates-containing exhaust gas or the like to flow from one end portion, filters the particulates by the partition walls, and discharge purified gas from the other end portion. However, there has been a problem that the particulates contained in the exhaust gas are deposited in one end portion (end portion on an inflow side of the exhaust gas) with the inflow of the exhaust gas to thereby close the cells. This is a phenomenon that easily occurs in a case where the exhaust gas contains a large amount of particulates, or in cold districts. There has been a problem that when the cells are closed in this manner, a pressure loss in the DPF rapidly increases. To inhibit the cells from being closed, it has been proposed that sectional areas of cells (inflow-side cells) opened in the end portion on the inflow side of the exhaust gas are set to be different from those of cells (outflow-side cells) opened in the other end portion (end portion on the outflow side of the exhaust gas). Here, the sectional area refers to the area of the section at a time when the cell is cut with the flat face vertical to the longitudinal direction.
However, when the sectional area of the inflow-side cell of the honeycomb filter including the cells having square sectional shapes is set to be different from that of the outflow-side cell, each partition wall forming the cell partially thins and weakens in each portion (hereinafter referred to also as the “intersection portion”) in which the partition walls cross one another. To solve this problem, post injection is performed when the PM is deposited on the DPF to thereby combust/remove the PM. However, in this case, there has been a problem that stress is concentrated on a part of the thinned intersection portion, and the part easily breaks. Here, the portion in which the partition walls cross one another refers to a portion belonging to both crossing partition walls in the section obtained by cutting the honeycomb filter with the flat face vertical to the longitudinal direction. For example, the portion refers to a square range of the intersection portion, when the partition walls linearly extending and having the equal thickness cross one another in the section.
Moreover, there has been a honeycomb filter having inflow-side cells having large sectional areas and outflow-side cells having small sectional areas, in which the sectional shape of the cell having the large sectional area is an octagonal shape obtained by linearly cutting a corner portion of the square shape (e.g., see French Patent Application Laid-Open No 2789327). This octagonal shape slightly solves the state in which the intersection portion partially thins, but a weakness problem still remains.